This invention relates to a pressure relief valve which is particularly suited for use in a fluid distribution system to reduce overpressure.
Current practice in the field of fluid distribution systems is to rely on direct acting or pilot operated pressure relief valves to avoid excess pressure developing within the system to avoid exposing attached components to damaging overpressures. Relief valves are normally connected to a xe2x80x9cTeexe2x80x9d in the system so that they discharge fluid to waste or a lower pressure zone when the system pressure exceeds the set point pressure.
Direct acting relief valves rely on a valve member that is exposed to an opening force created by the pressure of the fluid distribution system and a closing force created by a spring. If the pressure of the distribution system (the opening force) exceeds the closing force established by the spring, the valve will open to release fluid and relieve pressure until the spring force is sufficient to close the valve. Sometimes a dashpot is used to reduce fast closing or slamming of the valve.
Direct acting relief valves offer the advantages of opening virtually instantly and being useable in both clean and dirty water distribution systems. At the same time, such valves suffer from the disadvantage that very large overpressures are required to open the valve any significant amount. Once the seat area of the valve exceeds a few square inches, the forces necessary to open the valve must significantly exceed normal system pressure particularly when the valve is open. In general, very long springs are required with respect to the stroke of the valve so that the opening force necessary to move the valve does not increase greatly as the valve opens and the springs are compressed or extended. The opening force of the fluid acting against the moveable inner valve decreases significantly as the valve opens with the result that the pressure creating the opening force trends to the pressure in the portion of the valve immediately downstream of the seat.
These factors require that there be a substantial increase in system pressure before the direct acting valve opens beyond 10% of its stroke. A severe overpressure surge requiring discharge corresponding to greater than 50% opening of the valve results in high system overpressure before the relief valve permits sufficient flow to waste to limit further pressure increase. In view of the above design considerations, direct acting relief valves larger than approximately 2 inches in seat diameter tend to be large, clumsy and expensive with bulky housings and long springs As an alternative, pilot operated relief valves can be used to relieve overpressures in a fluid distribution system. Pilot operated valves employ a small direct acting valve to control a larger diaphragm or piston operated main relief valve. The pilot operated valves require needle valves, strainers or filters in the pilot circuit. This characteristic makes them suitable for clean water applications, but inappropriate for systems handling sewage or dirty water in which particulate material can clog the pilot circuit.
Pilot operated relief valves also suffer from the disadvantage that they are relatively slow to react to overpressures. Because pilot opera ted valves have to release fluid from a control chamber through tubing and the pilot valve itself , several seconds may be required for the valve to open fully. Booster valves may be used to speed opening, but the time delay may still be overly long to suit the operating specifications of the fluid distribution system.
To address the foregoing problems in prior art relief valve designs, we have developed a new pressure relief valve that offers the quick response time and clean/dirty water functioning of direct acting valves and the large fluid flow capabilities of pilot operated relief valves.
Accordingly, the present invention provides a pressure relief valve for relieving excess pressure in a fluid distribution system comprising:
a valve body having an inlet port and an outlet port in communication with an interior of the valve body, the inlet port being directly connectable to the fluid distribution system;
a valve member movable between a closed position to prevent fluid flow through the valve body and an open position to permit fluid flow through the valve body, the valve member being exposed to a first opening force exerted by the pressure in the fluid distribution system;
a movable member co-operating with the valve member such that the movable member is exposed to the pressure in the fluid distribution system to provide a second opening force for the valve member; and
biasing apparatus to exert a default closing force to move the valve member to the closed position, the closing force being equivalent to a pre-selected pressure in the fluid distribution system;
whereby the first and second opening forces act to move the valve member to the open position whenever the pressure in the fluid distribution system exceeds th e pre-selected pressure.
The valve unit of the present invention provides quick detection and opening of the valve member to relieve overpressures in the fluid distribution system.
The pressure relief valve finds application in both clean and dirty fluid applications. When used in a dirty water environment, an additional interface chamber is used to transmit the pressure of the fluid distribution system via a movable seal interface and an inert fluid to the movable member to exert the supplementary opening force.
Preferably, the area of the movable member is significantly larger than that of the valve member so that any system overpressure is applied to a combined area that is much larger than the relatively small area of the valve member to generate a larger opening force. This multiplication of the opening force allows the valve of the present invention to open fully with only a small increase in system pressure. The auxiliary chamber is isolated from the main fluid flow through the valve interior so the opening forces generated by the movable member tend to be unaffected by the pressure reduction that results as the fluid flows past the valve member.